The fabrication sequence of integrated circuits often includes several patterning processes. The patterning processes may define a layer of conductors, such as a patterned metal or polysilicon layer, or may define isolation structures, such as trenches. In many cases the trenches are filled with an insulating, or dielectric, material. This insulating material can serve several functions. For example, in some applications the material serves to both electrically isolate one region of the IC from another, and electrically passivate the surface of the trench. The material also typically provides a base for the next layer of the semiconductor to be built upon.
After patterning a substrate, the patterned material is not flat. The topology of the pattern can interfere with or degrade subsequent wafer processing. It is often desirable to create a flat surface over the patterned material. Several methods have been developed to create such a flat, or “planarized”, surface. Examples include depositing a conformal layer of material of sufficient thickness and polishing the wafer to obtain a flat surface, depositing a conformal layer of material of sufficient thickness and etching the layer back to form a planarized surface, and forming a layer of relatively low-melting point material, such as doped silicon oxide, and then heating the wafer sufficiently to cause the doped silicon oxide to melt and flow as a liquid, resulting in a flat surface upon cooling. Each process has attributes that make that process desirable for a specific application.
As semiconductor design has advanced, the feature size of the semiconductor devices has dramatically decreased. Many circuits now have features, such as traces or trenches, less than a micron across. While the reduction in feature size has allowed higher device density, more chips per wafer, more complex circuits, lower operating power consumption and lower cost among other benefits, the smaller geometries have also given rise to new problems, or have resurrected problems that were once solved for larger geometries.
An example of the type of manufacturing challenge presented by sub-micron devices is the ability to completely fill a narrow trench in a void-free manner. To fill a trench with silicon oxide, a layer of silicon oxide is first deposited on the patterned substrate. The silicon oxide layer typically covers the field, as well as walls and bottom of the trench. If the trench is wide and shallow, it is relatively easy to completely fill the trench. As the trench gets narrower and the aspect ratio (the ratio of the trench height to the trench width) increases, it becomes more likely that the opening of the trench will “pinch off”.
Pinching off a trench may trap a void within the trench. Under certain conditions, the void will be filled during a reflow process, for example where the deposited silicon oxide is doped and experiences viscous flow at elevated temperatures. However, as the trench becomes narrower, it becomes more likely that the void will not be filled during the reflow process. Moreover, several types of applications call for the deposition of undoped silicon oxide (sometimes referred to a undoped silicate glass, “USG”), which is difficult to reflow even at elevated temperature. Voids resulting from pinching-off are undesirable as they can reduce the yield of good chips per wafer and the reliability of the devices.
One possible solution to this problem is a two-step process wherein a first deposition step is performed under process conditions with a low (silicon-containing gas):(oxidizing gas) ratio and low deposition rate for achieving the desired gap-fill. After this first step is performed, the flow of silicon-containing process gas to the chamber is interrupted, and then a second distinct deposition step is performed under different conditions with a high (silicon-containing gas):(oxidizing gas) ratio and a high deposition rate for bulk fill. However, such a two-step process may exhibit reduced throughput due to the extended time required in the first step, and also the time consumed in halting the process gas flow and changing the apparatus configuration between the first and second steps.
Therefore, it is desirable to be able to fill narrow gaps with dielectric material in a void-free manner. It is also desirable that the process used to deposit the dielectric material be efficient, reliable, and result in high yield of devices.